A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around 40 cm2/Vs up to 67 cm2/Vs, resulting in a resistivity of 1.4× 10-4 ωcm has been obtained for annealing at temperatures of 650 °C. The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc oxide and the glass substrate and the silicon capping layer on top of the zinc oxide. Both are hydrogen-rich if deposited by plasma-enhanced chemical vapor deposition. For longer annealing times a decrease in carrier concentration can occur if a thin capping layer is used. This can be explained by the deteriorating effect of oxygen during thermal treatments which is well known from annealing of uncapped zinc oxide films. The reduction in carrier concentration can be prevented by the use of capping layers with thicknesses of 40 nm or more. © 2010 American Institute of Physics.